1. Field of the Invention
The invention relates to a method and an apparatus for generating a periodic training signal, particularly for an XDSL transceiver.
2. Description of the Prior Art
In modern telecommunication, data transmission at high bit rates on a subscriber line is playing an increasingly large role, since ever more applications, such as video or online gaming, require bi-directional data communication at ever greater data rates.
For transmitting digital data via a transmission channel, a series of modulation techniques for coding and modulating analogue signals which are routed to a reception apparatus via the transmission channel are known.
A technique which is recently becoming more and more significant is “multicarrier data transmission”, which is also known as “Discrete MultiTone” (DMT) transmission or as “Orthogonal Frequency Division Multiplexing” (OFDM) transmission. Multicarrier transmission is suitable particularly for data transmission via transmission channels which add linear distortion. In comparison with single-carrier data transmission, advantages also arise with regard to the flexibility when matching the data rate or the transmission spectrum to the transmission channel or to the disturbance environment. Multicarrier transmission is used in conductor-based systems, for example, but also in the radio sector, for broadcast systems and for accessing data networks. Examples of DMT transmission applications are digital broadcast radio (DAB=Digital Audio Broadcast) and digital television (DVB=Digital Video Broadcast), or alternatively mobile radio applications.
An example of a representative of DMT transmission is ADSL technology, ADSL standing for “Asymmetric Digital Subscriber Line”, that is to say the digital asymmetric subscriber line via a normal telephone line. ADSL denotes a technology which allows a bit stream at a high bit rate to be transmitted from a control centre to a subscriber and allows a bit stream at a low bit rate to be transmitted from the subscriber to a control centre. This technology makes use of the telecommunication line for conventional telephone services (that is to say voice transmission) and at the same time for data transmission. Besides ADSL technology, there are also further representatives of “xDSL technology”, for example the broadband subscriber line (VDSL=Very High speed Digital Subscriber Line), where the frequency range for data transmissions extends up to 17 MHz. There are also plans for VDSL2 applications, which use even higher frequency ranges.
DMT transmission systems use a multiplicity of carrier frequencies, with the data stream to be transmitted being broken down into a large number of parallel substreams which are transmitted using frequency-division multiplexing. These substreams are also called single carriers. For modulation, the transmitted signal is made up of a large number of sinusoidal signals, with every single sinusoidal signal being modulated both in terms of amplitude and in terms of phase. This produces a multiplicity of quadrature-amplitude-modulated (QAM) signals. For DMT transmission, Inverse Fourier Transformation (IFT) is used in the transmitter and Fourier Transformation (FT) is used in the receiver. Fast and efficient signal processing algorithms exist both for the IFT and for the FT.
A problem which accompanies very broadband DMT transmission, in particular, arises as a result of the fact that the usable frequency range in VDSL extends up to 17 MHz and in planned VDSL2 applications extends up to 30 MHz. However, this wide frequency range also contains frequency bands for other services, such as the frequency bands for medium-wave and short-wave broadcast radio or the frequency ranges for amateur radio. Depending on the physical position, the subscriber terminals for the DMT transmission may be subject to interference from these services which use the same frequency range as the DMT transmission. This is referred to as “RFI interference”, RFI standing for “Radio Frequency Interference”. This RFI interference is narrowband interference for the very broadband frequency range used for DMT transmission, since these interfering RFI frequency ranges have a width of typically just a few kilohertz.
If a narrowband interfering signal of this type is overlaid on the DMT received signal, this has a negative effect on the demodulated DMT received signal. In this case, interference arises not just for the values (carrier frequencies) in the direct vicinity of the centroid frequency of this interference. Carrier frequencies (or user channels) for the DMT received signal which are at a much greater distance from this interfering frequency are also subject to interference.
Besides bandwidth-limited interfering signals, the transmitted modulated signal is also attenuated, linearly distorted and can be overlaid by other extraneous signals. Particularly crosstalk interference may arise when an adjacent line pair or a physically adjacent transmission channel is likewise used to transmit DMT signals. In such a case, the interference takes effect through crosstalk of the entire bandwidth for the DMT signal. Since xDSL, for example, involves a large number of line pairs being routed in a form bundled next to one another, a plurality of sources of crosstalk interference normally arise at the same time, and these have an adverse effect on a modulated analogue signal which is to be transmitted. In such an FEXT (Far End Cross Talk=crosstalk at the other end of the line) environment, it is desirable to take measures to improve the signal quality of the relevant received signals.
In addition to the sources of crosstalk interference and RFI interference, which are relatively easy to describe, the received signal is also usually subject to stochastic noise.
FIG. 1 shows a DMT transmission system based on the prior art.
Digital data DS to be transmitted are first of all supplied to an encoder CD via a serial-parallel converter SPW. Depending on the quality or on a measured signal-to-noise ratio for the single carriers in the multicarrier data transmission, a configuration for a QAM symbol is chosen for each single carrier. The configuration stipulates the number of bits which can be transmitted by a QAM symbol. Thus, a particular number of bits can be transmitted for each single carrier using a QAM channel, which can be represented by a respective complex frequency value. FIG. 2 shows the configuration for a 16 QAM encoder.
The encoder CD associates the data block to be transmitted, which has a particular number of bits, with the single carriers as QAM symbols. The relevant frequency values of this data block for transmission, or else called DMT symbol or DMT frame, can be combined to form a complex frequency vector D (fμ) FIG. 3 schematically shows the spectrum of a DMT signal modulated with N carrier signals.
In this case, the frequencies fμ correspond to the respective single-carrier frequency. A typical VDSL transmission method uses, by way of example, N=4096 carriers at a respective distance of Δf=4.3125 KHz from one another, which exhausts a frequency range or transmission bandwidth of 0-17664 MHz. This frequency vector D(μ), where μ subsequently indicates the frequencies fμ of the single carriers, is subjected to Inverse Fourier Transformation, which is illustrated by the function block IFFT in FIG. 1. The Inverse Fourier Transformation provides a real signal vector in the time domain with M=2·(N+1) components. FIG. 4 shows a block diagram of a conventional IFFT unit based on the prior art.
A buffer buffer-stores the real signal vector, which is expanded by a “guard interval” GI (cyclic prefix). The guard interval GI provides a cyclic expansion for the DMT frame. The guard interval GI fills the intervals of time at which DMT frames are successively sent. The signal which has been encoded from the digital data and subjected to Inverse Fourier Transformation has a spectrum with a periodicity of the relevant inverse sampling frequency T=1/fA.
The digitally filtered data signal to be transmitted is converted to analogue using a digital-analogue converter DAW and is bandwidth-limited by an analogue filter AF1.
This analogue bandwidth-limited transmitted signal A(t) is supplied to a line driver LT for amplification and is then passed to an appropriate transmission channel CH. In the case of xDSL, this is a two-wire telephone line, for example. Interfering influences which can adversely alter the modulated analogue signal are illustrated in FIG. 1 as an additionally injected noise signal R(t) as a model. This noise signal R(t) is injected additively.
At the receiver end, the modulated analogue received signal Z(t) which is subject to interference is first of all subjected to analogue filtering AF2 and is then converted to a discrete-time digital received signal Z(tk) by an analogue-digital converter ADW at the sampling frequency fA. This digital discrete-time received signal Z(tk) is digitally filtered in a digital filter DF2, is then buffer-stored in a buffer and is divested of the guard interval GI. A device for Fourier Transformation FT is used to convert the received signal into a received signal vector E(fμ).
Without disadvantageous influences as a result of the signal processing and particularly as a result of the channel frequency response, the received signal vector E(fμ) provides precisely the same complex frequency values as have been produced at the transmission end by the Inverse Fourier Transformation IFFT. In a practical system, however, this is never entirely the case. To eliminate the influences of the frequency response of the transmission channel CH, a frequency range equalizer EF is provided which weights the complex entries in the received signal vector E(fμ) with equalization coefficients. These equalization or filter coefficients eliminate precisely the effects which are brought about by the transmission channel CH, which is characterized by its transfer function C(f). The equalization filter EF therefore has a filter function which corresponds to the inverse of the channel transfer function C(f). The equalization filter EF thus provides at least one partial signal improvement as a result of the compensation for the measurable channel effects.
The equalized received signal is processed in a decision-maker unit E which associates the frequency values which have the greatest probability of being transmitted with the corresponding received and equalized frequency values and decodes the single QAM signals. To recover the transmitted data DS, parallel-serial conversion PSW is then performed and is output as received data DE.
FIG. 5 shows the cyclic expansion of a DMT symbol. Following the Inverse Fast Fourier Transformation, each symbol comprises 2*N samples, where N is the number of carrier signals used in the DMT modulation method. For cyclic expansion, the first LCS samples are appended to the DMT symbol as a cyclic suffix and the last LCP samples are added before the DMT symbol as a cyclic prefix, as illustrated in FIG. 5. In the case of DMT, the channel is equalized using the guard interval which is formed by the cyclic prefix. By inserting this safety interval between two successive transmitted symbols, the channel memory is completely switched off, which means that the intersymbol interference ISI decreases. The cyclic suffix is required in the case of VDSL in order to perform “Digital Duplexing”. In the case of “Digital Duplexing”, the transmitted and received signals in the two transceivers are shifted relative to one another such that no symbol limit in the transmitted signal interferes with the FFT of the received signal. This means that a VDSL system operating “digital duplexing” does not require an echo cancellation unit.
In addition, VDSL transceivers perform windowing of the cyclically expanded DMT symbol, as illustrated in FIG. 6. In this case, a predetermined number (β) of samples from the start and at the end of the cyclically expanded DMT symbol is multiplied by a window function in order to form the spectrum for the transmitted signal. This involves the sidebands of the DMT signal being lowered, so that interference for other frequency bands is minimized. The lowering of the sidebands in the case of VDSL is necessary in order to observe the permitted power spectrum density. In ADSL transceivers, no windowing of the cyclically expanded DMT symbol is performed.
In the case of transceivers based on the VDSL standard or the VDSL-2 standard, the cyclic continuation of the discrete multitone signal is switched on right at the start of the training phase. Switching on the cyclic continuation or the cyclic expansion right at the start of the training phase allows the complexity of the hardware of the VDSL transceiver to be kept relatively low, since it is not necessary to alter the data transmission rates at a later time. In addition, switching on the cyclic continuation during the actual training phase allows the transmitted signal to be constantly overlaid with or multiplied by a window function, so that the sidebands are lowered.
However, the result of switching on the cyclic continuation right at the start of the training phase in conventional VDSL transceivers is that the cyclically continued DMT signals can be used to train the equalizers and other adaptive signal processing blocks at the receiver end only with difficulty and with poor performance.
FIG. 7A shows a training signal which comprises a sequence of cyclically expanded training symbols, each cyclically expanded training symbol comprising a cyclic prefix (CP) and a cyclic suffix (CS).
FIG. 7B shows the associated time signal for a very simple example in which the signal is modulated merely with a carrier signal. The carrier signal is sinusoidal. As can be seen from FIG. 7, the boundaries between the cyclically expanded training symbols, i.e. times t0, t1, t2, t3, have signal discontinuities or sudden amplitude changes which prevent optimum setting of the equalizers. For digital duplexing, however, the provision of a cyclic prefix (CP) is indispensable in VDSL transceivers.
In ADSL transceivers based on the prior art, the equalizers and the other adaptive signal processing blocks are able to be set by virtue of the cyclic continuation not being switched on at the start of the training phase. Only at a later time is the cyclic continuation switched in within the training phase in ADSL transceivers. This is possible because ADSL transceivers have an echo cancellation unit and do not use digital duplexing. In addition, a window function is not normally used to lower the sidebands. This is normally done in ADSL transceivers using digital filters. Changeover from a training phase without cyclic continuation to a training phase with cyclic continuation increases the circuit complexity within ADSL transceivers substantially, however.